For many reasons, controllable objects such as machines and devices need to be protected against unintended use by unauthorized third parties. These controllable objects may be power plant controls, sensitive production line controls or safety-relevant devices in hospitals. There are many controllable objects that require protection against unintended access. State-of-the-art protection schemes are typically based on single-factor authentication (e.g. the use of a key or a password) or two-factor authentication (e.g. the use of a card and a password). Incidents have been reported were an authorized user authenticated towards a controllable object and subsequently left the controllable object unattended without logging off. In such a case the controllable object becomes vulnerable against unauthorized manipulation. Furthermore, in harsh environments (e.g. explosion-endangered areas) expensive keyboards or touch screens are normally required to provide user access for data entry.